In many environments where electric motors are used to do useful work, an object is moved by the motor a predetermined distance. In such an event, often the motor is moved against a fixed limit of motion and the detection of the reaching of such limit needs to be sensed so that the operation of the motor can be ceased. In many prior art systems using direct current motors, the status of a direct current motor is typically monitored by mechanically measuring the torque of the motor or by electrically measuring the current drawn by the motor. Mechanical torque measuring systems tend to be cumbersome and expensive for applications where control of many small inexpensive direct current motors is desired. The use of a current monitoring technique for motor control status inquiry is somewhat cheaper but is subject to inherent inaccuracies because of the temperature and line voltage variations inherent in such a system. Another technique often used to avoid both of these systems is to utilize limit switches responsive to actual positioning of the object to send an electric signal when a limit of travel of the object is reached. Systems utilizing such limit switches are adequate and practical, but the use of such switches adds to the cost and complexity of the system with many motors since separate limit switches must be installed and wired for each motor control device. The present invention attempts to offer a different approach toward this purpose in that the only connection between the control circuit and the motor is the pair of power lines by which the motor is powered, and the back electromotive force on those lines is measured to determine when the limit of travel of the motor is reached by sensing the termination of this back electromotive force from the motor.
The prior art is generally cognizant of the concept of operating motors with microprocessor or digital control, with control being responsive to the position of the motor control device. For example in U.S. Pat. No. 4,431,954, a microprocessor controls motor operation for windshield wipers with the position of the windshield wipers at any given time being determined by position sensors. Similarly U.S. Pat. No. 3,792,332, provides an interface for multiplex motor control systems in which a microprocessor or other control box is used to operate a number of motive devices. Other window or closure operating systems are illustrated in U.S. Pat. Nos. 3,781,622 and 2,994,525.
The broad concept of sensing the electromagnetic force from motor operation as a means of controlling motor operation is not new in and of itself. For example, the disclosures of U.S. Pat. Nos. 4,119,899 and 4,358,718, disclose motor operating devices for direct current motors which are, to some degree, responsive to the counter electromotive voltage induced in the direct current motors. Both of these systems operate with conventional direct current voltage supplies.